Carburization of ferrous metal parts in molten salt baths has been known for many years. The conventional method involves using a substantial amount of a cyanide salt in a molten chloride bath. Although metal parts treated in the cyanide baths exhibit a high degree of surface hardness when quenched, the difficulty of safe handling and waste disposal have presented severe problems.
Many attempts have been made to develop non-cyanide carburizing processes. For example, Freudenberg, U.S. Pat. No. 1,796,248, describes a process using a mechanically agitated fused chloride salt bath with soda and finely divided carbon to introduce carbon into ferrous metal parts immersed therein. Leininger et al., U.S. Pat. No. 2,568,860, describes a similar bath using fused chloride and a carbonate; and instead of mechanical agitation, carbon monoxide or a gas forming carbon monoxide is bubbled through the bath. Further, Newell, U.S. Pat. No. 3,488,233, describes the use of molten lithium carbonate as the active carburizing ingredient. However, these methods using carbonates, generally tend to de-carburize initially and require a long interval of induction, or heating at high temperatures.
Other types of non-cyanide carburizing processes are described in Holt U.S. Pat. No. 2,049,806; Muller, U.S. Pat. No. 3,194,696 and Jakubowski et al U.S. Pat. No. 4,268,323. These processes involve the addition of organic nitrogen compounds such as urea, cyanates and dicyanodiamide. Such baths introduce both carbon and nitrogen into the treated parts, and in many applications, nitriding is not desired.
Other non-cyanide carburizing baths using metal carbides in molten salt are described in Albrect, U.S. Pat. No. 1,992,931; Solakian, U.S. Pat. No. 2,249,581; Steigerwald, U.S. Pat. No. 2,254,328 and British Patent No. 1,223,952. Among the metal carbides described, silicon carbide is preferred for good carburization. However, silicon carbide reacts with alkaline salts to form a silicate which is corrosive to steel and further gives rise to the objectionable formation of sludge or scum.
A further method is described in Leininger U.S. Pat. No. 2,492,803 which uses boron or silicon oxide in combination with carbonates and carbon to achieve carburizing. However, this method suffers from the same disadvantages as methods using silicon carbide.
More recent attempts to carburize by a non-cyanide liquid process are described in Foreman et al., U.S. Pat. No. 4,153,481 and Fox et al., Canadian Patent No. 944,665. Both of these patents describe processes using molten chloride and carbonate salt mixtures and graphite. However, as in prior attempts using carbon in the form of finely divided graphite, mechanical agitation is needed to disperse the graphite into the molten salt. This requires the modification of existing equipment used in cyanide baths, and requires large capital expenditures. Further, as stated previously for carbonate baths, a long interval of induction is required before carburization can by effected. For example, Foreman et al. U.S. Pat. No. 4,153,481 disclosed in Example 1 that about 5 hours of induction is needed. For the above reasons, cyanide processing is still the generally used molten salt carburizing method despite its many obvious disadvantages.
Therefore, it is the object of the present invention to develop a non-cyanide molten salt bath capable for producing a uniform depth of carbon casing free of nitrogen on ferrous metal surfaces.
Another object of the present invention is to provide a non-cyanide carburizing process capable of carburizing at a rate equal to or faster than the conventional cyanide process.
A further object of the invention is to provide a carburizing process employing readily available materials which are economical, require no special handling and create no waste disposal problems.
Yet another object of the present invention is to provide a carburizing process which can utilize equipment currently employed in cyanide processing.